rockchip: spl: rk3399: spi: enable SPL_SPI_LOAD if SPI is enabled for SPL

To include the ability to load from an SPI flash in SPL, it's not
sufficient to define SPL_SPI_SUPPORT and SPL_SPI_FLASH_SUPPORT via
Kconfig... so we conditionally define SPL_SPI_LOAD if SPI support
is already enabled for SPL via Kconfig.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>